If you've ever had to use this software, I'm sorry. It is not friendly. You essentially have to write your own SOAP server that is called by Quickbooks.
Then you have to send them XML in their QBXML format (of which, the documentation only really works in IE, kind of works in Firefox, and not at all in Chrome -- that's right, just reading the documentation requires a specific browser). Debugging is a pain.
So I thought a service that abstracts that would be a good idea. We would provide the Quickbooks WebConnector file to you, you install it, and then start sending your invoice information to a nice RESTful API.
We handle the rest and ensure your data gets into Quickbooks properly. If you're interested in something like this, please subscribe to the mailing list.
PS. I'm doing the same for the Google AdWords API, which is also not a joy to use -- http://metricsapi.com/tour/google-adwords-rest-api